1. Field Of The Invention
This invention generally concerns pavement cutting apparatus and particularly relates to an electro-hydraulic control system for accurately controlling the engagement and disengagement between a rotating cutting tool and a material being cut.
2. Description Of Prior Developments
A problem has long existed with concrete and pavement cutting saws and the like which use electro-hydraulic systems for raising and lowering a rotating blade. Saw operators prefer to complete their cutting operations in as little time as possible in order to increase their productivity and profitability. As a result, the saw operators wish to lower and raise the cutting blade as quickly as possible at the beginning and end of each cutting path.
Unfortunately, this practice has led to the destruction of many blades and has shortened the life of many engines and transmissions which power the blades. The primary reason for this component failure is the rapid lowering of the blade into the pavement. When the rotating cutting blade is hastily lowered into contact with the pavement the blade engine may easily become overloaded and stall. In addition, the blade may become overstressed so that its cutting elements are sheared away thereby destroying the blade.
Prior blade lowering units have commonly used a battery driven electric motor to power a pump which in turn energized a hydraulic cylinder. The hydraulic cylinder would typically raise the cutting blade and associated blade driving machinery upon receiving pressurized fluid from the pump via an operator-controlled check-and-dump valve. The check-and-dump valve used was either completely open to flow or completely closed to flow so that no control over the rate of blade movement was possible. The blade would be lowered at about the same rate it was raised.
Because an operator usually desires to raise the blade at a rate faster than the rate at which it is lowered, designers have incorporated an additional adjustable valve in the hydraulic line communicating with the cylinder. This adjustable valve included a variable restriction valve arranged in parallel with a one-way check valve. The adjustable valve was set at a fixed setting which determined the maximum rate of blade descent. The check valve would close during the lowering of the blade so that all fluid would pass through the set restriction. However, upon raising the blade, the fluid would flow in the opposite direction so that the check valve arranged in a parallel flow path with the variable restriction would open allowing a rapid actuation of the cylinder. In this manner a rapid raising or lifting of the blade was achieved along with a slow blade descent.
Unfortunately, this solution did not prove entirely satisfactory to the industry since the rate of blade descent, once set, was essentially fixed. In order to reset the adjustable restriction the operator had to access the valve which usually was located in an inconvenient location. Such adjustment was, for practical use, fixed in one position due to its inaccessability.
An unmet need expressed by many operators was for a control which would allow an initial rapid descent of the blade up to a point where the blade closely approached the cutting surface, and then for a slower descent to gently engage the blade with the pavement. This would minimize both raising and lowering time yet prevent damage to the blade, the blade power train, and the blade motor.
One solution to this problem involved the addition of yet another adjustable valve to the hydraulic system. An additional hydraulic line was connected between the reserve or return tank which stores and provides hydraulic fluid to the pump motor and a point in the main fluid line between the check-and-dump valve and the variable restriction valve. A parallel return line was then formed which allowed the variable restriction valve to be set at a relatively higher flow rate than prior systems to enable a more rapid lowering of the blade. As the blade approached the pavement, the operator turned the needle valve in the manner of a rotary faucet valve to further adjust (decrease) the rate of blade descent.
While this system performed generally satisfactorily, it was nevertheless inconvenient for the operator to manipulate since two valves had to be manually controlled, first the check-and-dump valve and then the needle valve. When the blade was adequately lowered using this known system, the operator had to close the needle valve completely to stop the blade descent. This usually distracted the operator and generally was considered as an annoyance, particularly in those cases where numerous short cuts were required.
Accordingly, a need exists for a blade lowering and raising system which may be completely controlled through a single actuator. A need also exists for a failsafe system which protects the blade and motor regardless of operator error.